Some mobile devices are configured to operate using more than one Radio Access Technology (RAT). These are known as multi-mode devices. Carriers or service providers that provide services in both 2G (such as GSM [Global System for Mobile Communication] EDGE [Enhanced Data Rates for GSM Evolution]) and 3G (such as UMTS [Universal Mobile Telecommunications System]) and possibly other advanced networks such as EPS (Evolved UTRAN (Universal Terrestrial Radio Access Network) and the Evolved Packet Core) may prefer for subscribers to access 3G networks or other advanced networks whenever feasible.
While a multi-mode mobile device in idle mode is in a 2G cell, a network may provide information on cells or frequencies with other radio access technologies (excluding E-UTRA) in the 3G Cell Reselection list (see 3GPP TS 44.018 Section 3.4.1.2.1.7). For a multi-mode mobile device that supports E-UTRA, E-UTRAN frequencies may be included in the E-UTRAN Cell Reselection list (see 3GPP TS 44.018 Section 3.4.1.2.1.7a). The 3G Cell Reselection list and the E-UTRAN Cell Reselection list are provided in a System Information Type 2quater (S12Q) message.
If the network provides information on UTRAN cells and possibly E-UTRAN frequencies associated with zero or more not allowed physical layer cell identities (which may be referred to as blacklisted cells) for cell reselection, measurement and reporting, this information is sent in the SI2Q message (see 3GPP TS 44.018 section 3.2.2.1). SI2Q messages are transmitted on a periodic basis and a mobile device with UTRAN or E-UTRAN capability (i.e. a multi-mode device) is configured to listen to the SI2Q messages to obtain a list of 3G cells and possibly E-UTRAN frequencies associated with zero or more not allowed physical layer cell identities which it should measure (or not use in the case of not allowed cell identities). The mobile device measures the neighbour 3G cells or the E-UTRAN frequencies provided and if at least one of those measured cells or frequencies passes threshold measurements as described in 3GPP TS 45.008 section 6.4, the mobile device reselects the 3G or E-UTRAN cell that rates the highest based on the measurements. If priority information is to be used for cell reselection (see 3GPP TS 44.018 section 3.4.1.2.1.12), which is signalled by the presence of the 3G Measurement Control Parameters Description IE or by the presence of the E-UTRAN Measurement Parameters Description IE in the S12quater message, then the Serving Cell Priority Parameters Description IE is also present in the SI2quater message in order to provide the serving cell priority and reselection parameters. If the MS receives any absolution priorities in dedicated messages (for instance when a call or data session is released) then these dedicated priorities override the priorities received in the SI2quater broadcast message.
If the network sends a list of cells associated with frequencies that are not allowed for reselection (e.g. a blacklist) then this information is also taken into consideration in the cell reselection procedure.
The list of neighbour cells or frequencies provided in the SI2Q message is not always accurate. Sometimes the backend data used to determine the neighbours is incorrect. As well, environmental conditions can cause errors in determining viable 3G cells or E-UTRAN cells. In such situations, timely reselection of the 3G network or the E-UTRAN network is inhibited. This is very visible to the user, as many multi-mode mobile devices will display on a user interface an indication of which RAT is being used.
In addition the list of neighbour cells or neighbour frequencies provided by the 3G network or E-UTRAN network, such as the neighbour cells provided in System Information Block Type 11, 11bis or 12 (see 3GPP TS 25.331 section 10.2.48.8.14, 10.2.48.8.14a, 10.2.48.8.15) may be inaccurate.